feat: device enumeration, VAD/PTT input gate, stereo playback, WASAPI loopback

Closes the three items PROGRESS.md's M3 section explicitly carried forward as
out of scope:

- Device enumeration (vc_list_devices) + input device selection
  (vc_set_input_device), backed by AudioEngine::enumerate_devices() via
  miniaudio's ma_context_get_devices. Device ids are opaque hex-encoded
  ma_device_id strings.
- VAD/PTT send-side input gate (vc_set_input_mode, vc_set_push_to_talk).
  webrtc-audio-processing (the originally-planned APM) has no working
  Windows/MSVC build upstream (GCC-only Meson, unfinished MinGW support, hard
  abseil-cpp dependency), so VAD is a new lightweight, dependency-free
  energy/RMS processor (EnergyVadProcessor) behind the existing ApmProcessor
  interface. Gating is MIC-only; SCREEN_AUDIO/AUX_DEVICE always bypass it.
- True stereo playback: AudioEngine's mixer and output device now carry
  stereo end-to-end (mono streams upmix L=R) instead of downmixing decoded
  stereo streams to mono before mixing.
- Real WASAPI loopback capture for SCREEN_AUDIO (Windows-only, via
  miniaudio's loopback device type), replacing test-only injection as the
  production capture path.

Also: vccli gains --list-devices, --input-device, --input-mode, and
--share-screen-audio flags, plus a stdin command loop (ptt on/off, mode
vad/ptt) for manual verification. New test_vad_ptt_devices.cpp covers all
four items (ABI-level + a white-box AudioEngine stereo-mix check).

Docs updated to match: voice.md, roadmap.md (decision-log entry superseding
the original webrtc-audio-processing choice), tech-stack.md, README.md,
architecture.md, CLAUDE.md, PROGRESS.md.

Still explicitly out of scope, documented not silently dropped: real
webrtc-audio-processing/AEC (no AEC/NS/AGC exists at all yet), macOS/iOS
SCREEN_AUDIO capture, process-specific loopback, and a pre-existing
RT-thread rule violation in the capture path that predates this work.

Verified: ctest 12/12 green across 3 consecutive full-suite runs (both dev
and m1-dev presets build clean); test_vad_ptt_devices passed 5 consecutive
standalone runs; manually verified live (vccli --list-devices against real
hardware, vccli --voice --input-mode vad streaming without incident).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_smoke.cpp

@@ -60,9 +60,17 @@ int main() {
     vc_result rc_join = vc_join_channel(c, 1, nullptr);
     CHECK(rc_join == VC_ERR_NOT_IMPLEMENTED || rc_join == VC_ERR_NOT_CONNECTED);
 
-    vc_device_list dl;
-    CHECK(vc_list_devices(c, VC_DEVICE_INPUT, &dl) == VC_ERR_NOT_IMPLEMENTED);
+    vc_device_list dl{};
+    vc_result      rc_devices = vc_list_devices(c, VC_DEVICE_INPUT, &dl);
+#ifdef VOICECAT_HAS_AUDIO
+    // Real device enumeration is wired up once miniaudio is linked in (post-M3 follow-up).
+    // Never assert count > 0 here — a headless CI build agent may legitimately report zero
+    // audio devices; only that the call itself succeeded.
+    CHECK(rc_devices == VC_OK);
+#else
+    CHECK(rc_devices == VC_ERR_NOT_IMPLEMENTED);
     CHECK(dl.count == 0);
+#endif
     vc_free_device_list(&dl);
 
     vc_client_destroy(c);